1. Field of the Invention
This invention relates to a process for the manufacture of electrodes comprising a layer of activated metal such as Raney nickel deposited on a base metal such as iron, cobalt or nickel and to the electrodes manufactured by the process.
2. Description of the Prior Art
Some industrial electrolysis processes, such as alkaline electrolysis of water or chloro-alkali electrolysis, require as a cathode an electrode with low hydrogen overvoltage. In addition, for alkaline electrolysis of water an anode is desirable which allows oxygen generation without overvoltage. For this reason, catalytic-acting electrodes are used.
The best-known catalyst for hydrogen generation is platinum. However, because of its high price only very thin platinum coatings are deposited on the electrodes, which are typically less than 1 mg Pt/cm.sup.2, so that the efficiency of the electrodes is not entirely satisfactory. As demonstrated by comparative tests conducted by Teledyne Energy Systems (Hydrogen Energy Progress-IV, Editors T. N. Veziroglu, W. D. Van Vorst, J. H. Kelley, Pergamon Press, Oxford, 1982, pages 151-158), catalysts based on non-noble metals are overall more favorable.
One example investigated by Teledyne was a nickel/molybdenum catalyst developed by the BP Research Centre in Middlesex (EP Patent Application No. 79 301 963.9). Nickel boride catalysts (DE-PS No. No. 2 307 852) were also tested, which yielded higher hydrogen overvoltages than platinum coatings (Hydrogen Energy Progress III, Editors T. N. Veziroglu, K. Fueki, T. Ohta, Pergamon Press, Oxford, 1981, pages 15-27). Other known catalysts include nickel sulfide (BE-PS No. 864 275) and various coatings with mixed transition metal oxides (Seminar on Hydrogen as an Energy Vector, EEC Report EUR No. 6085, 1978, pages 166-180) or transition metals (GB No. 1 510 099 and U.S. Pat. No. 4,152,240).
Hydrogen overvoltage is effectively reduced by the use of Raney nickel electrodes (E. Justi, A. Winsel, "Cold Combustion, Fuel Cells", Franz Steiner Verlag, Mainz, 1962). However, these electrodes, originally produced using a molding process, can only be manufactured in customary industrial sizes uneconomically and with difficulty. Therefore, an improved rolling process was developed by Lurgi (J. Mueller, K. Lohrberg, H. Wuellenweber, Chem.-Ing.-Techn. 52 (1980) pages 435-436), which makes possible a simple enlargement of the working surface of the electrodes. According to this process, nickel or steel sheet is clad with Raney nickel powder, and activated in the customary manner by treatment with KOH. In addition to a low overvoltage, the excellent long-term behavior of electrodes produced in this manner is remarkable. After an operating time of one year at a current density of 2 kA/m.sup.2 and an operating temperature of 92.degree. C., the electrode potential was unchanged.
Similar long-term behavior is also exhibited by electrodes coated with Raney nickel, which can be obtained exceptionally easily be activation of a galvanically-deposited Ni/Zn layer (J. Divisek, H. Schmitz, J. Mergel: Chem-Ing.-Techn. 52 (1980), page 465). See also German Pat. No. 1 294 943 which shows that from a solution containing Ni.sup.2+ and Zn.sup.2+ ions, a Ni/Zn alloy is galvanically deposited on a electrode matrix and is then activated in the customary manner to Raney nickel with an alkali solution.
The electrode obtained in this manner can be used as a cathode in alkaline generation of hydrogen or as a cathode and/or anode in alkaline electrolysis of water. The matrix used must have a good electrical conductivity and can have various geometric shapes, so that there is no "a priori" limitation on the design of an electrolyzer. Preferred geometric shapes of such an electrode matrix are wire meshes, metal meshes or perforated sheets. The last-named form is especially important, since it makes possible the so-called "sandwich" construction of an electrolysis cell for alkaline electrolysis of water, in which the two electrodes are directly in contact with the gas separator (diaphragm, ion conductor) with "zero distance" between them, so that the distances between electrodes are minimized and the ohmic voltage drops become negligible.
3. Problems of the Prior Art
Certain problems occur in the activation of thin nickel electrodes, specifically with perforated sheets. Usable depositions of Raney alloys, especially of Ni/Zn alloys from an electrolyte containing Ni.sup.2+ and Zn.sup.2+ ions, require cathode current densities of 4-7 A/dm.sup.2, or more. At the same time, the potential over the total surface area of the electrode matrix to be coated with the activatable alloy must be as uniform as possible, i.e. for a uniform, controlled deposition, the potential differences within the cathode caused by the ohmic voltage drop may not exceed 40 mV. However, such potential differences do occur within the cathode surface with low sheet thicknesses of the electrode matrix of about 0.2-0.5 mm, such as those which are preferred for alkaline electrolysis of water, on account of the lower price and technological advantages over sheet thicknesses of 1 mm and more.
Thus, for example, during the galvanic Ni/Zn coating of an industrial-size electrode matrix for alkaline electrolysis of water of 2-4 m.sup.2, electrical currents are on the order of 1000-3000 A during the galvanic deposition. With such current strengths, in order to be able to conduct the galvanic deposition with a potential difference in the electrode of less than 40 mV, the Ohmic resistance in the sheet must be kept correspondingly low. For this purpose, either the sheet thickness of the electrodes must be increased, which entails considerable technological disadvantages and increased costs, or the current path through the electrodes must be kept correspondingly short during the galvanic deposition. With larger electrode surfaces, that can only be done by means of a number of contact points or lines. Only then does the galvanic deposition become uniform and reproducible and can, by subsequent treatment with KOH, be converted into an activated electrode layer (Raney nickel layer) for chloro-alkali electrolysis or alkaline electrolysis of water.
Moreover, inside the electrolyzer for alkaline electrolysis multiple contacts over the electrode surface are also required. As a rule these contacts are not the same as the ones described above, so that the contacts used for the coating must be removed once again and new ones applied. Defects thereby occur on the surface of the electrodes, which interfere with uniformity and therefore power. If, on the other hand, during the galvanic deposition of the nickel alloy, pressure contacts are provided for the current distribution, then uncoated areas occur at the pressure points, which subsequently also interfere with the electrolysis.
Therefore, a uniform defect-free and controlled galvanic Ni/Zn coating of thin electrodes with industrial dimensions can not be achieved in the customary manner. The same is true for electrodes based on Co and Fe, which are activated by coating with an alloy of electrode base metal and metal which can be leached by a leaching treatment, such as tin or zinc, and a subsequent leaching of these components.